The present invention relates generally to manufacturing and, more particularly, to the use of a derived metric for monitoring die placement.
Semiconductor die are normally formed in large quantities on wafers of semiconductor material, for example, silicon. After die are singulated from the wafers, they may be individually packaged in plastic or ceramic packages, for example. A lead frame may support the die for wire bonding and packaging and provide the lead system for the completed package. In general, electrical circuitry formed on the die is coupled to bond pads on the die to facilitate interconnection of the electrical circuitry with the outside world. During the wire bonding and packaging process, each bond pad is electrically connected by way of wire leads to the lead frame. The electrical connection includes a wire bond formed on the bond pad, a wire lead and a wire bond formed on the lead frame. An encapsulating material protects and insulates the die, and the die is mounted in a package having external pins for interconnecting the electrical circuitry on the die, via the wire bonds, to the outside world.
A packaging tool places the die on the package for attachment thereto. The package typically includes reference marks, commonly referred to as fiducial marks, for use by the packaging tool. Following the placement of the die, a placement measurement tool determines, on a sampling basis, the accuracy of the placement by measuring the rectangular offsets between the actual center of the mounted die and expected center point (i.e., with perfect alignment) and an angular offset resulting from rotation of the die with respect to the package. The placement measurement tool thus outputs an x-offset, h, a y-offset, k, and an angular offset, θ, as illustrated in FIG. 1, where the actual die 100 is illustrated using solid lines, and the ideal die placement 110 is illustrated using phantom lines. The degree of misalignment is exaggerated for illustrative purposes.
The results reported by the placement measurement tool are typically trended using statistical process control (SPC) techniques, such as control charts. Each type of device manufactured in a facility may have a different size. Hence, the actual displacement of the corners of the device, where misalignment between the bond pads and the lead frame is most likely to cause packaging faults, is not readily apparent given the center and rotation measurements. A typical fabrication facility processes devices of differing types on multiple fabrication lines. Due to the large number of product sizes, different packaging tools, and different placement measurement tools, the amount of die placement data generated is extremely large. Given the volume of the data and the differing size characteristics of the packaged die, it is sometimes difficult for facility operators or automatic management systems to identify performance problems with a particular placement tool by trending the offset data.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the present invention described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the present invention. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art. The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.